Normally, natural gas contains only very small amounts of sulfur (S). It is assumed that the sulfur content in the combustion gas for a gas turbine is completely converted to sulfur dioxide (SO2) during the combustion process with oxygen (O2).S+O2→SO2 
In the gas flow of the heat recovery steam generator, sulfur dioxide is partially further converted to sulfur trioxide (SO3):SO2+½O2→SO3.
The resulting sulfur dioxide—and sulfur trioxide concentrations in the exhaust gas are in any case initially comparatively small. Nevertheless, the sulfur trioxide concentration should be determined or at least estimated with regard to possible sulfuric acid formation and its negative effects on the cold end of a heat recovery steam generator of a gas and steam turbine plant, because sulfur trioxide reacts with water (H2O) to give sulfuric acid (H2SO4):SO3+H2O→H2SO4 
The sulfuric acid condenses on falling below the sulfuric acid dew point and leads to corrosion. A determining or estimation of the sulfur trioxide content is not entirely simple, however, because the conversion rate is a function of various parameters.
For example, in units with catalysts for the selective catalytic reaction or with catalysts for carbon monoxide, the sulfur dioxide conversion increases at these catalysts; the sulfur trioxide concentration is therefore increased.
A further factor which is to be taken into consideration is the dwell time of the exhaust gas in the heat recovery steam generator, which fluctuates depending on a gas turbine load. The lower the gas turbine load, the longer the dwell time of the exhaust gas also is in the heat recovery steam generator and consequently the conversion of sulfur dioxide to sulfur trioxide also increases.
A provision for the prevention of sulfuric acid is to ensure that the temperature of the condensate preheater in the heat recovery steam generator lies permanently above the sulfuric acid dew point.
In the interests of as high an efficiency of the plant as possible, the temperature of the condensate preheater should, however, be as low as possible.
It is therefore common practice for gas and steam turbine plants to carry out an estimation of the sulfur dioxide conversion rate, which will be rather more conservative with regard to corrosion problems and at the expense of efficiency. For this, the minimum temperature of the waste gas in the heat recovery steam generator of a gas and steam turbine power plant is selected, through corresponding design of the heat recovery steam generator and a correspondingly high condensate flow temperature, to be so high that sufficient reliability is present in order to in any case not fall below the acid dew point of the sulfuric acid (e.g. 10 K over the expected dew point). Through the “safety distance”, the exhaust gas in the heat recovery steam generator is not cooled to an extent as would be theoretically possible without the occurrence of sulfuric acid corrosion. Hereby, if applicable, heat is “given away” in the region of several MW, the efficiency of the gas and steam turbine power plant reduces. It would therefore be desirable to be able to determine the sulfur trioxide content in the exhaust gas more precisely.
The sulfur trioxide content or respectively sulfuric acid content of the exhaust gas can take place through sampling and chemical analysis in the laboratory. However, such a method is slow, laborious and expensive [Continuous Measurement Technologies for SO3 and H2SO4 in Coal-Fired Power Plants, EPRI, Palo Alto, Calif.: 2004. 1009812.].
The sulfur content in coal leads, in coal-fired power plants, to sulfur trioxide concentrations in the exhaust gas of 1 to 100 ppm. For this concentration range, probes exist which determine the sulfuric acid content in situ by the method of controlled condensation [Continuous Measurement Technologies for SO3 and H2SO4 in Coal-Fired Power Plants, EPRI, Palo Alto, Calif.: 2004. 1009812.].
For the same concentration range, there is also an approach of measuring in situ by means of laser spectroscopy directly in the exhaust gas [http://practices.geosyntec.com/air-quality/pdf/Geosyntec-SO3-Analyzer-Technical-Brief.pdf]. Extractive methods with laser spectroscopy are also known. The methods are, however, limited hitherto to comparatively high sulfur trioxide concentrations (0.5 ppm-200 ppm) [http://www.psicorp.com/pdf/library/sr-1210.pdf], [http://www.ayt.cl/files/articulos/WP_AQISO3_0810.pdf], [U.S. Pat. No. 8,368,896 B1].
The sulfur trioxide concentrations which are expected in the exhaust gas of a gas-operated power plant lie in the range of 10 to 1000 ppb.